1366X768 helps those who want to play on their convertibles. Also - strange but true - quite a few elder users hate high resolutions. To them, smaller resolution = larger fonts, easier to read. I know you can increase font size on high resolutions, but tell that to your 70-year granny.
Why do they even bother to offer 1366x768 resolutions anymore? 1080p should be the standard. If they can offer an 8GB RAM, 256GB SSD model that is not too expensive I can see it being very popular. All depends on the cost.